Surveying instrument and method

ABSTRACT

A tacheometer utilizes a gallium arsenide laser diode in a phase shifting distance encoder, a sinusoidal interpolator in a phase shifting shaft angle encoder, a mercury pool in a phase shifting two axis off-level encoder, a shared phase to digital decoder and a digital processor, to measure angles and distances corrected for off-level, speed of light variations, refraction and the earth&#39;s curvature.

TABLE OF CONTENTS

Reference to Related Patents

Background and Summary

Description of the Drawings

System Architecture

Keyboard

Theodolite

Level Sensor

Distance Module

Balance and Beam Break Circuitry

Phase Detector

Processor and Display

Accumulator and Input/Output Module

Optics

Detailed Sequences

REFERENCE TO RELATED PATENTS

This application is related to the subject matter of U.S. Pat. No.3,619,058, entitled DISTANCE MEASURING APPARATUS issued Nov. 9, 1971 toWilliam R. Hewlett and Gregory Justice and to the subject matter of U.S.Pat. No. 3,900,259, entitled TIME INTERVAL PHASE DETECTION IN DISTANCEMEASURING APPARATUS issued Aug. 19, 1975 to Claude M. Mott and RichardJ. Clark.

The digital processing modules utilized in this application are relatedto the subject matter of U.S. Pat. No. 3,863,060, entitled GENERALPURPOSE CALCULATOR WITH CAPABILITY FOR PERFORMING INTERDISCIPLINARYBUSINESS CALCULATIONS issued Jan. 28, 1975 to Frances Rode et al and tothe subject matter of U.S. Pat. application entitled ADAPTABLEPROGRAMMED CALCULATOR HAVING PROVISION FOR PLUG-IN KEYBOARD AND MEMORYMODULES, filed Dec. 26, 1972 by Freddie W. Wenninger et al.

BACKGROUND AND SUMMARY

Tacheometers, the generic term for theodolite/distance measuringinstrument combinations, are powerful surveying tools capable ofmeasuring the angles and the distances between points. Typically, thisis done by leveling the instrument, optically aligning a gimbaledtelescopic sight upon a target, measuring the shaft angles of the sightonce aligned, and measuring the distance between the instrument and thetarget. Shaft angles can be measured by graduated vernier techniques orby more advanced digital encoding techniques described in more detail inthe section below entitled THEODOLITE. Techniques for referencing theinstrument to level are also described therein. Techniques for measuringdistances include parallax methods as well as phase shift techniquessuch as described in more detail in the section below entitled DISTANCEMODULE. The major drawback of the prior art in tacheometers has beenthat a small, lightweight, and highly accurate tacheometer hasheretofore been unobtainable.

A preferred embodiment of the present invention has a photosensitivereceiver positioned to receive external and internal light beams forproducing first and second electrical signals respectively in responsethereto. A limit detector is coupled to the receiver via a variable gainamplifier for generating first and second range signals in response todetecting the amplified first and second electrical signals attainingvalues outside of first and second ranges. The first range,corresponding to operable limits of the intensity of the external lightbeam, is larger than and includes the values of the second range. Thesecond range corresponds to a desired dynamic range of the electricalsignals. A balancing circuit, coupled to the receiver and positioned forvarying the intensity of one of the light beams, causes the value of thesecond electrical signal to follow the average value of the firstelectrical signal. Since the value of second electrical signal followsthe value of the first and has a smaller range, if the external lightbeam intensity varies gradually, a second range signal is generated. Inresponse to a second range signal, the variable gain amplifier iscoupled to a feedback loop for adjusting the amplified electricalsignals to values within the limit detector ranges and distancemeasurements are continued. If the external light beam intensity attainsa value outside of its operating limits, a first range signal isgenerated. In response to a first range signal, the variable gainamplifier is left uncoupled from the feedback loop, the balance circuitis inhibited and no distance measurement is made. The instrument is thusprepared to make distance measurements immediately upon the externallight beam re-attaining a value within its operating limits.

In operation, the preferred embodiment of the present invention can makeaccurate distance measurements upon external light beam signals havinggradually varying intensities. Further, the present invention identifiessudden losses of the external light beam and places the instrument in ahold mode of operation awaiting for the return of the external beam. Thepreferred embodiment is thus capable of making continuous and accuratedistance measurements in a variety of adverse operating conditions.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a prespective view of a tacheometer constructed in accordancewith the preferred embodiment of the present invention.

FIG. 2 is a block diagram of the electronics within the tacheometer ofFIG. 1.

FIG. 3 is an illustration of the keyboards and display of thetacheometer of FIG. 1.

FIG. 4 is an illustration of the encoder disc used in the shaft angleencoders of the tacheometer of FIG. 1.

FIGS. 5A-5C illustrate the optical layout of photo sources, detectorsand the encoder disc of FIG. 4.

FIG. 6 is an illustration of the sinusoidal track and detector of theencoder disc of FIG. 4.

FIG. 7 is a graph of light intensity as a function of angle received bythe diode elements of the detector of FIG. 6.

FIG. 8 is a detailed schematic of the sinusoidal track decoder.

FIGS. 9A & 9B are detailed block diagrams of FIG. 2.

FIG. 10 is an illustration of the radial slit track detectors of FIG. 5.

FIG. 11 is a flow chart for combination of the angular measurements.

FIG. 12 is an illustration of the eccentricity error measurement.

FIG. 13 is an illustration of the optical layout of the level sensor ofthe theodolite of FIG. 9.

FIG. 14 is a detailed illustration of the detector and source assembliesof FIG. 13.

FIG. 15 is a detailed block diagram of the distance module of FIG. 2.

FIG. 15a is the waveform of the output from the receiver of FIG. 15.

FIG. 15b is a detailed block diagram of the laser control circuitry ofFIG. 15.

FIG. 15c is a detailed schematic of chopper 310 and the motor controlcircuitry of FIG. 15.

FIG. 15D is a detailed schematic of the balance and beam break circuitryof FIG. 15.

FIG. 16 is a graph of the transfer function of the receiver diode ofFIG. 15.

FIG. 17 is a block diagram of the phase detector of FIG. 2.

FIG. 18 is a block diagram of the processor and displays of thetacheometer of FIG. 2.

FIGS. 19A & 19B are block diagrams module of FIG. 2.

FIG. 20 is an illustration of transmitter and receiver optics of thedistance module of FIG. 15.

FIG. 21 is an illustration of telescope optics of the distance module ofFIG. 15.

SYSTEM ARCHITECTURE

A tacheometer constructed in accordance with the present invention isillustrated in FIG. 1. The tacheometer is mounted upon a tripod 10 andcan be leveled thereon by levelers 15. The operator first sights thetarget (a cube reflector) through spotting scope 20 and then sights thetelescope 25 upon the target through eyepiece 30. Vernier alignment canbe made with the vertical vernier 35 and the horizontal vernier 40, andlocked in place with vertical and horizontal locks 45 and 50. Theoperator activates the instrument by switching on a power switch fromthe auxiliary keyboard 55 and selects the desired measurement sequencesvia the keyboard 60. The outputs are then displayed on output display65.

A block diagram of the tacheometer electronics is shown in FIG. 2. Thedistance module 75 transmits a modulated light beam downrange to a cubereflector which reflects the beam back to the instrument. The phaseshift between the transmitted and received beams is proportional to thedistance between the reflector and and instrument. This phase differenceis measured by the phase detector 80 and accumulator 85. Horizontal andvertical angles are measured by the theodolite module 90. Angles areencoded as phase shifts and also measured by the phase detector 80 andthe accumulator 85. The input/output module 95 interfaces the processor100 with the measurement modules. Measurement sequences are controlled,measurements are accepted, and logical flags are interrogated by theprocessor. Keyboard 60 provides a control interface with the processor100 by which the operator may select various measurement sequences andprocessor calculations. The output appears on the display 65.

KEYBOARD

The tacheometer keyboard 60, the auxiliary keyboard 55, and the outputdisplay 65, are illustrated in FIG. 3. The tacheometer has two keyboards60 and two output displays 65, on opposite sides of the instrument, withonly one active pair at one time. The auxiliary keyboard 55 comprisesswitches for selecting an output display of distance converted to feetor meters and angles converted to degrees or grads. The "COMP" switch isused to selectively activate processor compensation of distances andangles for an off-level condition sensed by the 2 axis level sensor,described below in the section entitled LEVEL SENSOR. Power switches forthe instrument and the telescope graticule illumination are on thesecond row of the auxiliary keyboard 55 as is the "PPM" dial forselecting a parts-per-million correction factor to compensate forchanges in the velocity of light caused by changes in the index ofrefraction of air due to variations in the air temperature and pressure.The PPM potentiometer is shown coupled to the environmental correctionmultistable multivibrator (one shot) 455 in FIG. 17.

Refer now to the 12 key keyboard 60, in FIG. 3. The "DIR" key (#6)measures the angle of the telescope 25 with respect to the horizontalcircle (θ) in the theodolite module 90 in FIG. 2. If the COMP switch onthe auxiliary keyboard 55 is activated, the processor will correct thereading for out of level. The "RD" key (#9) measures the relativedirection by substracting the last DIR reading from the current reading.This allows an operator to sight on a reference point, push the DIR key,then sight on a second point, push the RD key, and obtain the horizontalangle therebetween. Key number 7 measures the vertical angle (φ) in amanner similar to the function of the DIR key. The vertical measurementis also corrected for an off-level condition when the COMP switch is on.Key number 3 measures the slope distance. The distance module 75sequences through 3 modulation frequencies. Outputs from these arelogically combined to give a readout in either feet or meters on theoutput display 65. Key number 4 measures the projected distance. Thetacheometer measures the slope distance, the vertical angle (φ), and thelevel angles if the COMP switch in on, then calculates the projecteddistance. Key number 5 is the elevation difference key. This keyperforms a sequence similar to the projected distance key but calculatesthe elevation difference. The projected distance key and the elevationkey also correct for earth curvature and refraction. This is requiredbecause the gravity vector is not parallel for distant points. The trackkey "TRK" takes periodic readings from any of the numbered keys. Forexample, by pushing TRK, then key 3, the tacheometer will measure theslope distance about once a second so that a slowly moving target can betracked. Key number 2 reads out both level angles simultaneously ineither seconds or centicentigrads. Two three-digit numbers are displayedside by side. It is possible to very accurately level the instrumentwhile tracking this function, or simply verify that the instrument issafely within its limits and let the tacheometer compensate internallyfor off-level condition. Key number one displays the signal strength,obtained from the AGC one shot 450 illustrated in FIG. 17 and discussedin the section entitled PHASE DETECTOR, and the PPM correction dialed infrom the PPM control on the auxiliary keyboard 55, side by side. The PPMcorrection is adjusted by tracking this function while adjusting the PPMpot to obtain the desired correction. Key number 8 is a self-testselector. The tacheometer self checks a set of internal functions anddisplays all eights on the output display 65 if the test is completedsatisfactorily. The basic functions, keys 1, 2, 3, 4, 5, 6, 7, and 9,have dedicated storage locations in memory where the last measurement isstored. For instance, pushing key 4 will measure the level angles, slopedistance and vertical angle, then calculate projected distance andelevation difference. By now keying the recall key "RCL" followed by key2, 3, 5, or 7, the component measurements used in the projected distancemeasurement can be recovered. The output key "OUT" is used to send theoutput to a peripheral device. An HP9815 calculator described incopending U.S. Pat. application Ser. No. 597,957 entitled PROGRAMMABLECALCULATOR filed July 21, 1975 by Bradley W. Miller et al, can beinterfaced with the output interface 70 shown in FIG. 1.

The output display 65 has two indicator lights on the right hand side ofthe display. The DIST light is on only if a signal is being returned tothe distance module, indicating that a distance measurement is inprogress. If too strong a signal is being received, the light will alsofail to light, indicating to the operator the need to fit an attenuatorcap over the reflector. The level sensor light (LVL) flashes if theinstrument is outside the range limits of the level sensor and will beon continuously when within range. This light therefore functions as apilot light also. The first digit of the display (the left hand display)displays the key number of the function processed when an output isdisplayed to identify the output.

THEODOLITE

The function of the Theodolite Module 90 of FIG. 2 of the presentinvention is to measure the vertical angle phi (φ) and the horizontalangle theta (θ) of the telescope 25 relative to the reference plane ofthe tacheometer. It is desirable to have a theodolite which is small insize, highly accurate and compatable with digital electronics. Previoustheodolites, such as those described in U.S. Pat. No. 3,541,572 issuedto V. G. Shults, Nov. 17, 1970 and U.S. Pat. No. 3,675,238 issued toKarl Heinz Butscher on July 4, 1972, are less than optimum solutions tothese three requirements. Our theodolite uses two shaft angle encoders,one for the horizontal angle and one for the vertical angle, incombination with a 2 axis level sensor which is used to preciselydetermine the gravitionally defined level reference plane and totransform the vertical and horizontal measurements obtained from thevertical and horizontal shaft angle encoders to the gravitational plane.The two shaft angle encoders are identical in construction. Eachconsists of a flat transparent plate (the encoder disc) coated with aoptically opaque metallic film defining a plurality of circular tracksthereupon, each track having predefined optical characteristics. Theencoder disc 115 is illustrated in FIG. 4. There are three types ofcircular tracks upon the encoder disc 115; the digital tracks 120, thesinusoidal track 125, and the radial slit track 130. As illustrated inFIGS. 5A, 5B, and 5C, light is emitted on one side of the encoder disc,transmitted through the circular tracks upon encoder disc 115, anddetected upon the opposite side of encoder disc. The decoding of thesignals transmitted through the digital tracks 120 gives a roughmeasurement of the angle to be measured. By logically combining thisoutput with the output detected from the sinusoidal track and the radialslit track this angular measurement is interpolated in two stages togive a final angular resolution of less than 1 arc second.

The eight digital tracks 120 upon disc 115 utilize a Gray Code which iswell known to persons skilled in the art of theodolites. This code ischaracterized by having only one transition per one bit of information.The use of eight separate photo-transistors, denoted as photo-transistorarray 135 in FIG. 5A, produces a direct 8 bit Gray Code which can bedirectly interfaced with a conventional digital processor, giving anangular resolution of approximately 1.4°.

The sinusoidal track 125 (of FIG 4) is a 128 period transparent tracksinusoidally modulated in width. The detector 140 illustrated in FIG. 5Bcomprises four rectangular photo-sensitive diodes, D1-D4, upon a singlechip spaced 90° apart in respect to the period upon the sinusoidal track125. FIG. 6 illustrates the relationship between the detector 140 andthe sinusoidal track 125. The utilization of the four diode array upondetector 140 allows the use of a differential ratio technique whicheliminates offset and gain errors.

The intensity of the light received by the four diodes in the detector140 is illustrated in FIG. 7. The eight track digital system describedabove unambiguously defines the period of the 128 track sinusoidalpattern to be interpolated. The interpolation angle, φ₁, to be measured,is related to the intensities detected by the four diode detectors indetector 140 by the following relationships:

    I.sub.1 = I.sub.0 + sin φ.sub.1

    I.sub.2 = I.sub.0 + sin (φ.sub.1 + π/2) =  I.sub.0 + cos φ.sub.1

    I.sub.3 = I.sub.0 + sin (φ.sub.1 = π) = I.sub.0 - sin φ.sub.1

    I.sub.4 = I.sub.0 + sin (φ.sub.1 + 3π/2) = I.sub.0 - cos φ.sub.1

Referring now to FIG. 8, the outputs I₁ and I₃ from the first and thirddiodes in detector 140 and the oututs I₂ and I₄ from the second andfourth diodes in detector 140 are differenced in amplifiers 145 and 150respectfully.

    I.sub.1 - I.sub.3 = 2 sin φ.sub.1

    I.sub.2 - I.sub.4 = 2 cos φ.sub.1

The sinusoidal track photo source 155 (in FIG. 5) is modulated with asignal proportional to sinωt, (375Hz), thus the two signals output fromamplifiers 145 and 150 are:

A sin φ₁ sin ωt and

A cos φ₁ sin ωt₁ respectively.

Referring now to FIG. 9, the sinusoidal track decoder 160 illustrated inFIG. 8 is now shown interconnected within the theodolite module 90. Afirst output signal from sinusoidal track decoder 160 is coupled tocapacitor 165. The capacitor introduces a 90° phase shift to the signal.The second signal output from the sinusoidal track decoder 160 iscoupled to resistor 170, which matches the impedance of capacitor 165but introduces no phase shift. The two signals are subsequently summedat summing node 175. The phase shift and the summing result in an outputsignal which is proportional to:

    A sin (ωt + φ.sub.1).

The phase difference between the signal (sin ωt) modulating thesinusoidal track photo-source 155 and the signal at the summing node 175(sin (ωt +φ₁)) is therefore directly proportional to the angle φ₁, theinterpolation angle desired. In the current embodiment, the drivermodulation of the sinusoidal track photo-source 155 is a 375Hzsquarewave. Since the squarewave introduces high order harmonics, twoband-pass amplifiers 180 and 185 are used to suppress the undesiredharmonics and retain only the desired fundamental frequency. The outputof band-pass amplifier 185 therefore is a sinewave shifted in phase byan amount proportional to the angular displacement to be measured,having a peak-to-peak voltage of approximately 2 volts. As can be seenfrom FIG. 9, the resistor 170, capacitor 165 and band-pass amplifiers180 and 185, are time shared between the various decoder assemblies inthe Theodolite Module 90.

A second order interpolation is implemented upon the radial slit track130 of FIG. 4. The technique is the same as described as used for thedecoding of the sinusoidal track 125, except in the case of the radialslit track 130, the circular track comprises 4096 bars and slits and thedetectors 190 comprise four photo-diodes each with a superimposedsinusoidal mask thereupon. As illustrated in FIG. 10, each detectordiode has a mask comprising 13 sinusoidal periods, such that the outputamplitude is averaged over 13 periods. This reduces the sensitivity todust and improves the accuracy since it averages out patternimperfections over the thirteen periods. The diode masks are shifted byincrements of 90° with respect to the period of one bar and space. Thediode elements D1-D4 are offset 90° apart in phase in respect to theradial slit track 130. The same detector is used as on the sinusoidaltrack 110, except for the sinusoidal overlay mask. Referring to FIG. 9,two radial slit decoders 195 and 200 are illustrated. These decoders arespaced 180° apart upon the radial slit track 130, and the average valueof the two readings is used to indicate the actual phase angle upon theradial slit track 130. The difference between the two outputs from theradial slit decoders 195 and 200 is also used to correct foreccentricity errors, described below.

The horizontal shaft angle encoder 205 illustrated in FIG. 9 isidentical in operation to the vertical shaft angle encoder 210 which hasbeen described above.

Referring to FIG. 9, the processor will sequentially enable the decodersand the corresponding photo-sources for the measurement it requires viathe `select` lines. The outputs from the Gray Code on thephoto-transistor arrays, such as photo-transistor array 135, will bedirectly output to the input/output section of the processor on datalines C1-C8. The outputs from the sinusoidal and radial slit decoderssuch as decoders 160, 195 and 200 will be multiplexed upon the sharedoutput to the phase detector 80. The algorithm for combining the GrayCode, the sinusoidal track information, and the radial slit trackinformation, is shown in FIG. 11. First the digital Gray Code is readand converted to a binary coded decimal. The two radial slit outputs areread, averaged and the eccentricity correction required is calculated.Next the sinusoidal track information is read, corrected for theeccentricity error detected above, and then combined with the radialslit track information and the Gray Code angle to unambiguously definethe shaft angle to a resolution of better than 1 arc-second.

Band-pass amplifiers 180 and 185 inherently introduce a certain amountof phase shift which may vary as a function of time and temperature.This shift is measured by introducing the 375Hz squarewave directly viareference amplifier 183 during the reference measurement cycle. Acorrection is applied to subsequent measurements by the processor.

Eccentricity errors can be caused by bearing clearance, misalignment ofthe encoder disc on the shaft and artwork errors on the encoder disc.The eccentricity effects that we are concerned about in the presentinvention are on the order of a 1 mil, where the outer track is only 3mils in period. A 1 mil uncertainty in the interpolation of thesinusoidal track may result in the incorrect selection of the properperiod on the radial slit track, so the sinusoidal track detection needsto be corrected for the eccentricity effects. We are mesuring this withthe fine resolution of the radial slit tracks. The eccentricity ismeasured by taking the difference between the two radial slit trackmeasurements. If there is no eccentricity, the measurements will beexactly 180° apart. If there is an eccentricity error, as illustrated inFIG. 12, the true center of the encoder disc 115 will be displaced by adistance d from the center axis of the detectors and the radial slittrack measurements will not be displaced by exactly 180°, but will bedisplaced by an angle approximately 2∝ smaller than 180°. On FIG. 12,extrapolating a line radially out from the center of encoder disc 115through the sinusoidal track detector 140 would result in an error ofdetermining the period of the radial slit track 130 to be interpolated.The angular displacement of the sinusoidal track detector 250 isapproximately 2∝, therefore correcting the measurement from detector 140by 2∝ results in a correction of the eccentricity error without therequirement for a second sensor on the sinusoidal track. This requires,of course, that the three sensors, the two for the radial slit track andthe one on the sinusoidal track, be linearly aligned. The calculationsand corrections for the eccentricity are all done by the processor as isthe combining of the angular measurements.

LEVEL SENSOR

The function of the level sensor 215, shown in FIG. 9, is to measure theangular orientation of the instrument relative to a gravitional definedlevel, so that the vertical and horizontal angles which have beendetected by the vertical and horizontal shaft encoders 205 and 210, canbe transformed to a gravitational reference plane. The mathematics forcorrecting the vertical and horizontal angles once the true level hasbeen determined by a 2 axis level sensor is a straight forwardapplication of spherical geometry. See for example the section of"Relations in any Spherical Triangle" in the Chemical Rubber Company'sstandard mathematical tables, 14th Edition, published in 1964 by theChemical Rubber Company. The use of a level sensor to correct thevertical and horizontal angles measured by a theodolite, as taught bythe present invention, furthers the objectives of the present inventionby allowing the theodolite to self compensate for out-of-levelconditions.

The accuracy of theodolites in the prior art is dependent upon theaccurate leveling of the theodolite assembly itself, using the prior artleveling techniques such as bubbles or pendulum methods such as thatdescribed in U.S. Pat. No. 3,617,131 issued to Hiromi Taguichi, Nov. 2,1971. The present invention uses a lens to collimate a beam of lightwhich is reflected from a mercury pool and then focused on a detectorassembly. The displacement of the source image upon the detectorassembly due to the gravitionally determined plane of the mercury poolprovides a measure of the angular displacement of the theodoliteassembly from the gravitional reference on 2 axes. Referring to FIG. 13,the source assembly 220 produces an image which is transmitted throughlens 225 and collimated by lens 230. The collimated source image isreflected from mercury pool 235 which provides a gravitationally definedreference plane. The reflected source image is thereupon transmittedthrough lens 230 and and lens 225 and focused upon the detector assembly235 which is symmetrically disposed in relation to the source assembly220 about the symmetrical axis of the lens system. The function of lens225 is to keep the dimensions of the instrument package small. Thedisplacement measurement technique is identical to that described abovein the discussion of the shaft angle encoders. FIG. 14 shows thedetector assembly 233 and the source assembly 220 constructed upon asingle plane. The source assembly 220 has three patterns thereupon. Twoof the patterns are on orthogonal axes for determing the level of theassembly. One is denoted the gamma axis pattern 240, the other isdenoted the beta axis pattern 245. A third pattern 250 is a rectangularpattern used to determine whether the level sensor is within the limitsof its detection capabilities. The present level detector has a range ofapproximately ±3 min. from true level. The detectors are symmetricallydisposed from the center axis of the lens assembly upon a radius ofapproximately 3mm relative to the source patterns on the source assembly220. The photo-transistor 255 will output a signal if the level sensoris within range limits. The beta axis and the gamma axis detectors 260and 265 will output an array of signals from which the beta and gammaangular displacement of the true gravity vector can be determined by themethod previously defined in the shaft angle encoder technique.Referring now to FIG. 9, the processor will select a measurement uponeither the beta or the gamma axis which will be decoded by decoder 270or 275. The photo-transistor 255 will output a signal upon the flagLVSFLG if the level sensor is within its limits. If a beta or a gammameasurement is desired, the 375Hz squarewave driver signal will beapplied to the photosource which will emit the appropriate sourcepatterns, and the beta or the gamma decoder assembly will be activated.The output will be multiplexed upon the output lines shared withhorizontal shaft encoder 205 and the vertical shaft encoder 210, throughresistor 170 and capacitor 165 and band pass amplifiers 180 and 185 tothe phase detector 80.

The mercury pool 235 of FIG. 13 is designed to produce a level andhighly reflective reference plane. A layer of transparent silicon oil239 having a low viscosity temperature coefficient (VTC) is used to dampvibrations. A glass window 237 over the mercury/oil pool eliminates anyair bubbles from existing between the glass surface and the pool. Anexpansion lid 243 allows for thermal expansion and contraction of thepool. Finally, the coated glass window 237 has a low reflectivity, ˜1.0%as compared to ˜4.0% for uncoated glass, to prevent stray reflectionswhich are detrimental to accuracy.

DISTANCE MODULE

The distance module 75 of the present invention utilizes a techniqueknown as phase comparison, which is the subject matter of U.S. Pat. No.3,619,058 issued to William R. Hewlett et al, Nov. 9, 1971. A morerecent implementation of this technique is described in an articleentitled "Electronic Total Station Speeds Survey Operations," by MichaelL. Bullock and Richard E. Warren in the April, 1976 issue of theHewlett-Packard Journal. The present distance measuring module comprisesa transmitter, a local oscillator, optics, balance and beam breakcircuitry, and a receiver. Transmitter 280 of the present invention,shown in FIG. 15, comprises a 15MHz crystal oscillator 285 and a seriesof dividers coupled thereto for producing reference frequencies of15MHz, 375KHz, 3.75KHz and 375Hz. The 375Hz is used to drive thesinusoidal track photo-sources and the radial slit track photo-sourcesin the theodolite module. One of the three higher frequencies isselected by the processor via `lion` line inputs to the frequency selectcircuitry 290 which are coupled to the processor. The selectedmodulation frequency is input to the laser control circuitry 295 formodulation of the Gallium-Arsenide laser diode 300. The laser diode 300generates a beam in two directions. One beam is transmitted out of thetransmitter module, the second beam is sensed by sensor 305, which isincorporated in a feedback control loop for controlling the operatingpower level of the laser. The outgoing beam goes through chopper 310which alternately sends the beam along external and internal paths. Theexternal path sends the beam to a cube corner 315 and back, then throughan interference filter 320 which passes only the selected infra-redsignal to the receiver 325. The internal path sends the beam through avariable density attenuator 330 and directly into the receiver 325. Thereceiver diode 335 in the receiver 325 is a photo-avalanche diode. Thereceiver diode 335 has a gain which is a function of the reverse biasvoltage applied to the diode. These characteristics are illustrated inFIG. 16. As shown, the receiver diode has a gain of approximately 1 atlow reverse bias voltages. Increasing the voltage bias increases thegain to approximately 1000 before the diode goes into breakdown. Thereceiver diode accomplishes three functions in the present embodiment.First, the receiver diode demodulates the infra-red beam. Second, thereceiver diode mixes the received signal with a local oscillator signalproduced by local oscillator driver 340. Third, the receiver diodeamplifies the incoming signal an average of about 75 times. The localoscillator frequency is derived from the local oscillator module 340 andis selected so that the output from the receiver diode will always havea 3.75KHz component. When the laser is modulated at 15MHz by thetransmitter module the local oscillator is driven at 3.75KHz above the15MHz laser modulation. When the laser is modulated at 375KHz the localoscillator is again driven at 3.75KHz above the laser modulation. Whenthe laser is driven at 3.75KHz no mixing is required to produce the3.75KHz output from the receiver diode.

The local oscillator module 340 produces the two required localoscillator frequencies by means of two phase-locked loops. The firstphase-lock loop comprises a voltage controlled oscillator 345 which willbe controlled to oscillate at 3.75KHz above the 15MHz reference. Theoutput from the voltage controlled oscillator 345 is mixed with the15MHz reference signal from the transmitter module in mixer 350 and thedifference frequency is compared in phase detector 355 with the 3.75KHzfrequency from the transmitter 280. The output of the phase detector 355is low passed filtered and input to the voltage controlled oscillator345 to lock the output of the voltage control oscillator at exactly thedesired frequency. The second phase lock loop comprises a voltagecontrolled oscillator 360 which is to be locked at a frequency 3.75KHzabove 375KHz. This is done by dividing the output of the phase voltagecontrolled oscillator 360 with a 101:1 divider 365. This dividedfrequency is locked to the 3.75KHz signal from the transmitter module inphase detector 370. The output of the phase detector 370 is low passedfiltered and is used to control the voltage controlled oscillator 360.The output of the voltage controlled oscillator 360 is thereby locked ata frequency exactly 101 times the 3.75KHz reference. The localoscillator frequency selector 372 is controlled by the processor toapply either the 15MHz + 3.75KHz frequency or the 375KHz + 3.75KHzfrequency to the local oscillator driver 375 in the receiver module 325.The 3.75KHz output of the receiver diode 335 is passed through a lownoise amplifier 380, low pass filtered in low pass filter 385 and inputto an automatic gain control amplifier 387 which insures that the outputvoltage of the receiver circuitry will always be about 2 voltspeak-to-peak, as is the output from the theodolite module. Two narrowband-pass amplifiers 390 and 395 insure that only the 3.75KHz componentsare output from the receiver module 325.

The peaks of the output of low noise amplifier 380 are sampled by twocomparators 381 and 383. If an overload signal is detected, such aswould be caused by holding a cube reflector directly in front of theinstrument, comparator 381 will reduce the D.C. bias on receiver diode335 to reduce the nominal gain to approximately 20. If the overload isstill in existence even with the reduced receiver gain, comparator 383inhibits the GOODFLG. This will turn off the DIST light on the outputdisplay indicating to the operator that an attenuator is required in thebeam path.

The processor accepts distance measurements, samples and averages thesample values and computes the variance from the value of the samplemean. If the variance is within limits, the processor will display thesample mean. If not, the processor will request more samples. If after160 samples on the lowest frequency the variance is still out of limitsthe reading is aborted and a flashing zero is displayed. The same testis made upon middle and high frequency samples, however the highfrequency is allowed 320 samples to come within the variance limits.

The laser control circuitry 295 is shown in more detail in FIG. 15b.

BALANCE AND BEAM BREAK CIRCUITRY

The balance and beam break circuitry 600 controls the intensity of theinternal beam, the accumulation of data, and indirectly controls theautomatic gain control. The output signal from the receiver 325 isillustrated in FIG. 15a. Envelope detector 605 measures the maximumamplitudes (the envelope) of the receiver output as shown in FIG. 15a.When gate 610 is active, the internal beam balance circuitry is enabled.Synchronous detector 615 samples the output of envelope detector 605 attimes synchronized with the beam switching. In turn, the synchronousdetector drives meter 620 to adjust the variable attenuator 330 toequalize the internal and external beam envelopes.

Limit detector 625 determines whether the internal and external beamenvelopes are within predefined limits. Upon detection of anout-of-limit condition a signal is transmitted to logic module 630. Thelogic module 630 controls the gate 610, the "GOODFLG," and interactswith the processor to control the AGCDISQ flag as follows.

Three types of beam breaks can occur, and we categorize these as a"fast" beam break, such as a speeding car momentarily breaking the beam,a "long" beam break, such as a cow grazing in the beam, or a "slow" beambreak, such as fog slowly attenuating the external beam. In the case ofa "fast" beam break the external beam goes outside of its limits, andthe "GOODFLG" signals the processor to ignore the affected measurementcycle. In the case of a "long" beam break the balance is held, however,the AGC is adjusted to center the internal beam within its limits. Whenthe grazing cow moves out of the beam the external beam will again bewithin limits and the "GOODFLG" will signal the processor to start ameasurement. In the case of a "slow" beam break the balancing circuitrycan equalize the internal and external beam strengths. The internal beamlimits have a smaller tolerance than the external beam so that if thefog continues to roll in and attenuate the signal the internal beamlimit is triggered. This results in the correction of the AGC gain andthe measurement is restarted. If the processor does not receive a"GOODFLG" in 10 seconds (3 if tracking) the instrument goes to a standbymode. This produces a 10% laser duty cycle on laser 300 which conservespower and laser lifetime. The different limits upon internal andexternal beams, the continued balancing of internal and external beams,the balance hold and AGC update during "long" beam breaks, and the AGCupdate on "slow" beam breaks provides an instrument with improvedmeasurement and tracking capabilities. A detailed schematic of thebalance and beam break circuitry and of the chopper 310 controlcircuitry is illustrated in FIG. 15C. Note that the 10Hz signalindicates whether an internal or external beam is presently beingprocessed and proper ranges are accordingly selected by limit detector625.

PHASE DETECTOR

As explained in the discussion of the theodolite module 90 and thedistance module 75, both the angle information and the distancemeasuring information are now encoded as phase shifts on a periodicsignal. A phase detector 80 as illustrated in FIG. 17 is constructed inaccordance with U.S. Pat. No. 3,900,259 entitled TIME INTERVAL PHASEDETECTION IN DISTANCE MEASURING APPARATUS issued to Claude M. Mott andRichard J. Clark, Aug. 19, 1975. A differential input from thetheodolite module 90 is input to limiter 400 to construct a squarewavefor comparison with a reference signal. The output received from thereceiver 325 is input to a low offset amplifier (a zero crossingdetector) 405 which is then coupled to limiter 410 to also produce asquarewave for comparison to an appropriate reference signal. Functionselector 415 is controlled by the processor to select either the inputfrom the distance measuring module or from the theodolite module. Thissame control also controls function selector 420 which selects theappropriate reference frequency, either 3.75KHz squarewave for themeasurement of distance or a 375Hz squarewave for the measurement ofangles. The signals from function selector 415 are then applied to acoincidence phase detector 425 to determine whether the phase angledetected is close to 360°. Since the distance measuring determination aswell as the level sensor angle determinations require an averaging of anumber of input signals, operating close to 360° phase shift mayintroduce an error due to averaging of signals from different cycles. Ifsuch is the case, the coincidence phase detector 425 will provide asignal through the processor through OR gate 430 and the accumulator 85,and an 180° phase shift will be introduced to the reference signal inmodule 435 to avoid any possible averaging errors. Subsequent to thedetermination of whether the 180° phase shift is required, the inputsignal from function selector 415 and the reference signal from functionselector 420 are input into the phase detector 440, which is simply aset-reset flip-flop. This will hold the accumulator gate high for aperiod of time proportional to the phase difference between the inputand the reference signals. Counter logic 445 counts the number of phasemeasurements which have been made and outputs a signal upon thedetection of the 100th phase measurement to the processor when measuringdistance or the first phase measurement when measuring angle. The AGCone shot 450, and the environmental correction one shot 455, can also beselected to input to OR gate 430. These function as simpleanalog/digital converters. These one shots output a pulse correspondingin length to the voltage applied thereto, which provides a convenientmeans for interfacing with the processor. The selection of which of thefour measurement is to be input to OR gate 430 is controlled by "LIONS"lines from the processor. Only one of the inputs to OR gate 430 isactive at a time.

PROCESSOR AND DISPLAY

The processor 100 and the displays 110 shown in FIG. 18 aresubstantially the same as those used in the handheld calculatorsdesignated the HP35 and HP80 manufactured by the Hewlett-PackardCompany, Palo Alto, Calif. These are described in U.S. Pat. No.3,863,060 entitled GENERAL PURPOSE CALCULATOR WITH CAPABILITY FORPERFORMING INTERDISCIPLINARY BUSINESS CALCULATIONS, issued to FranceRode et al on Jan. 28, 1975. A control and timing chip "C&T" 460 iscoupled to the keyboard 60 by a 7×5 line matrix. This limits the numberof possible keys to 35. In the present invention only 24 of the possiblekey functions are utilized, 12 on each of the two keyboards. Four quadread-only memories (ROMs) 463 are coupled in parallel to the C&T 460.Two cathode drivers 465 are used to drive the output displays 65. Theread-out select 470 determines which of the cathode drivers 465 isactive. Two arithmetic and register circuits 475 and 480 areincorporated. One is totally dedicated to run the readout display andanother is dedicated to computations. This allows the instrument to doits computations while maintaining a simultaneous display. A datastorage chip 595 contains 10 registers for storage of temporary andsemipermanent calculations. This allows the "RECALL" function to operateas described in the KEYBOARD section. Two registers are used as scratchregisters. Registers 1 and 6 are updated only by direct inputs. Theremaining six registers are erased prior to each measurement. Ingeneral, the register contents correspond to the key stroke numeral asshown in Table 1.

                  Table 1                                                         ______________________________________                                        Register      Contents                                                        ______________________________________                                        0             Scratch                                                         1             SIG/PPM                                                         2             Level Readings (2)                                              3             Slope Distance                                                  4             Projected Horizontal Distance                                   5             Projected Vertical Displacement                                 6             Direction (Horizontal Angle)                                    7             Zenith (Vertical Angle)                                         8             Scratch                                                         9             Relative Direction                                              ______________________________________                                    

The system architecture of the processor 100 and a detailed descriptionof the C&T circuit 460, the read-only memories 463, the A&R circuits 475and 480, the clock driver 485, the anode driver 490, cathode drivers465, and a supplemental description of the keyboard 60, the outputdisplays 65, and the instruction set for the processor is given in theabove mentioned patent issued to France Rode et al. The programsequences programmed into the 4 quad ROMs 463 control the instrumentoperation in response to keyboard inputs and the other instrumentinputs. Listings of these sequences as well as further discussion of theprocessor instruction set are provided in the section entitled DETAILEDSEQUENCES.

ACCUMULATOR AND INPUT/OUTPUT MODULE

The "grand central station" of the tacheometer is the input/outputmodule 495 of FIG. 19. This module provides the interface between theprocessor 100 and the measurement modules. This input/output module 95was originally developed for the HP9805 Desk Top Calculator to interfacewith a printer and is further described in the co-pending patentapplication entitled ADAPTABLE PROGRAMMED CALCULATOR HAVING PROVISIONFOR PLUG-IN KEYBOARD AND MEMORY MODULES, serial number 318,451 filedDec. 26, 1972 by Freddie W. Wenninger et al. The input/output module 85performs basically three functions. The module excepts the data from theaccumulator 85, controls the instrument via "lion" and "tiger" lines andinterrogates instrument status via "flag" lines.

As explained above, the basic instrument measurements are output by thephase detector 80 in the form of timed pulses. This output is applied tobuffer gate 495 to gate a 15MHz clock. A number of clock pulsesproportional to the duration of the time pulse from phase detector 80 ismeasured by the accumulator 85. When the timed pulse terminates a BCDnumber will be stored in the five-decade counters of the accumulator 85.Since the input/output module 95 can accept only eight input lines, theaccumulator output is multiplexed into the input/output module viamultiplexer 500, reading first the 8 least significant bits, then themost significant bits sequentially into the input/output module. Theinterface circuitry 505 is required to adapt the CMOS logic of thetacheometry circuitry to the T² L logic of the input/output module 95interface. The input/output module 95 now communicates with the C&T 460and A&R 475 for computations and sequence control. This allows data tobe entered electrically through the processor and A&R chip in additionto processor control by the keyboard 60.

Instrument control is accomplished via the 9 "lion" lines. The functionof the lions lines are shown in Table 1.

                  Table 1                                                         ______________________________________                                        LIONS 1-4                                                                             Control Decoder 510 For Selection Of Theodolite                               Measurements, Angle or Distance Selection, and                                Selects PROM Initialization Constants For Angles                              and Levels.                                                           LIONS 5-6                                                                             Transmitter Frequency Select and Selects PROM                                 Initialization Constants For Distance.                                LION 7  Peripheral and Display Control Line Selector.                         LION 8  Level Limit Indicator.                                                LION 9  PROM Enable For Distance Measurements.                                ______________________________________                                    

The lions lines 1-4 control the eight select lines to the theodolitemodule 90 via the 4 to 10 decoder 510 and select angle or distancefunctions. The frequency selector and the local oscillator selector inthe distance measuring module are controlled to two more lions lines.

The three "siberian tiger" outputs combined with the SCE line provideshort pulses which last less than one instruction cycle time. These areused for short controls or interrogations. For instance, the digitaltrack photo-source 200 in the vertical shaft encoder 210 of thetheodolite module 90 requires only short discharges of a capacitor forcurrent pulses through the Gallium Arsenide photo-source. Controlmultiplexers 515, 517 and 519 multiplex the tiger lines. Multiplexselector 523 enables one of the control multiplexers via lines M1-5.This provides an effective capability of controlling many `little` tigerlines. The functions of the `little` tiger lines thereby obtained aredescribed in Table 2.

                  Table 2                                                         ______________________________________                                        (Q means "latched") -                                                         Group #1 LITTLE TIGERS                                                        PCENQ      Phase Coincidence Detector Select.                                 PDENQ      Phase Detector Select.                                             AGCDISQ    ACG Disable.                                                       TGRP2Q     Tiger Group 2 Select.                                              PDEN       Phase Detector And Phase Coincidence                                          Detector Enable.                                                   ACUD       Accumulator Count Up Or Down.                                      Group #2 LITTLE TIGERS                                                        RPSQ       Reference Phase Shift To Phase Detector.                           TGRP1Q     Tiger Group 1 Select.                                              ACCPRST    Accumulator Preset (From PROM).                                    VDSEN      Vertical Digital Sensors Enables.                                  HDSEN      Horizontal Digital Sensors Enables.                                ACCINH     Read Higher Order Accumulator Bits.                                ACCINL     Read Lower Order Accumulator Bits.                                 ECOSEN     Environmental One Shot Enable.                                     RSOSEN     Return Strength (AGC) One Shot Enable.                             Group #3 LITTLE TIGERS                                                        DIS1EN     Display 1 Enable.                                                  DIS2EN     Display 2 Enable.                                                  PERIN      Peripheral Input.                                                  PERLD      Peripheral Load.                                                   PERCK      Peripheral Clock.                                                  SELFTEST   Self Test Activate.                                                ______________________________________                                    

The final communication between the input/output module and theinstrument is via the flag lines. An example of a flag line is the rangesensor line LVSFLG from the level sensor module 215 in the theodolite.The input/output module has the capability of interrogating only asingle flag line at a time. The control multiplexer 520 multiplexes thevarious flag lines into the input/output module flag input. The flaginputs are described in Table 3.

                  Table 3                                                         ______________________________________                                        Flags:                                                                        ______________________________________                                        EXTFLG    Chopper Syncronization Flag.                                        OVFLO     Accumulator Overflow.                                               ACCRDYFLG Measurement Complete (Read Into Processor).                         GOODFLG   No Beam Break Detected.                                             LVSFLG    Level Sensor Within Range.                                          DEGGRAD   From Auxiliary Control Panel.                                       FTMTR     From Auxiliary Control Panel.                                       ______________________________________                                    

The PROM 525 is used to provide offset constants for the distance andangle measurements. Once the instrument is assembled, the offsets aremeasured and programmed into the PROM 520. These are used to preset thedecade counter automatically prior to any distance or anglemeasurements.

The processor can provide a digital output of the contents of thedisplay plus certain status bits. This output can be manually activatedby pressing the output key or automatically activated by pressing thefollowing key sequence: TRK + OUTPUT + KEY 1,2,3,4,5,6,7,8 or 9. In themanual case a single reading will be output after each measurement ismade. The output is accomplished via a five wire interface. There aretwo flag lines, a ground line, a clock line and a data line. The dataconsists of 14 BCD digits in a 56 bit serial stream.

OPTICS

The optics incorporated in the distance measuring module are illustratedin FIGS. 20 and 21. Referring to FIG. 20, a beam of light is emittedfrom laser 300 and collimated by lens 530. Chopper 310 alternatelyinterrupts the internal and the external beams so that only one of thebeams arrives at receiver diode 335 at a time. The beams are split intoa reference beam and a transmitted beam by prism 545. The reference beamis focused by lens 535 upon mirror 540, recollimated by lens 535,reflected from the back surface of prism 545, and focused by lens 550upon the receiver diode 335. The transmitted beam from chopper 310 isreflected off the front surface of reflector 545. This beam iscollimated and will exist side by side with the collimated received beamwhich passes through filter 320 and is focused by lens 550 upon thereceiver diode 335. The side by side existence of the collimatedtransmitted and received beams allows this assembly to be interfacedconveniently with telescopes having various powers. The present 30×power telescope 25, illustrated in FIG. 21, comprises two sphericalcomponents. The second surface mirror 555, also called a Mangin mirror,is the main power of the telescope. A slightly negative lens element onthe second surface mirror 555 is used to correct for sphericalabberation from the spherical reflective surface. Doublet 560 is aconvergent meniscus lens comprising a biconvex element and a biconcaveelement, the biconcave element facing the second-surface mirror 555,both elements made out of material having the same index of refraction.The primary function of the doublet 560 is to correct for coma(off-axis) abberations. The doublet 560 has a reflective surface 565interposed between the lens elements. The doublet has a slightlypositive power. This slightly positive power provides color correctionfor the negative refraction power associated with the second surfacemirror 555. Since the main power of the telescope is in the mirrors, andvery little power is in the glass lenses, there is no significant colorabberation. The side by side transmitted receiver beam is reflected offthe beam splitter 570. A negative lens 575 collimates the beams forinterface with the optics of FIG. 20. The beam splitter 570 allows asignificant portion of the optical wave lengths to pass through thereand eventually into the operator eyepiece 30. A small positive lens 580increases the focusing range and allows the telescope to be focused to 5meters. The prisms 585 are used to revert and invert the image. Sincethe field of view is 1.5°, and the power of the telescope 30×, the fieldof view at the eyepiece is 45°. This requires the use of two doublets570 in the eyepiece to obtain sufficient off-axis correction thuskeeping the entire field in focus. The use of the concave Mangin mirrorhaving negative refraction element and the slightly positive convergentmeniscus lens provides a short telescope having good spherical, colorand coma correction, and having a large aperture. All of the opticalsurfaces are spherical which provides a simple to manufacture device aswell.

The telescope is gimbaled so as to have a "plunging" capability. Thisrefers to rotating the telescope through vertical to a position 180°horizontally from a first position without moving the horizontal shaft.This allows an operator to take two sights, forward and back, from asingle position and thereby compensate for any eccentricity in thevertical gimbal of the theodolite. This technique provides for everyhigh angular resolution when combined with the present instrument. Thisfeature is combined with dual keyboards to allow the operator to controlinstrument sequences during both measurements.

DETAILED SEQUENCES

A complete listing of all the routines employed by the processor isgiven below. All of the routines are stored in the four quad-ROMs. QuadROM 1 contains the keyboard and other general instructions. Quad ROM 2contains sequences pertaining to the theodolite module. Quad ROM 3contains sequences pertaining to the distance measuring module. Quad ROM4 contains the self-test and service test routines. Each quad ROM issubdivided into four sections. The four sections of quad ROM 1 aredesignated 10, 11, 12, and 13. Designations of the other quad ROMs aresimilar. In the complete listing below, each section has 256instructions identified by line number in the first column. The secondcolumn contains the octal address. The third column contains an optionaladdress for branch instructions. The fourth column gives the binary bitpattern of the operation code. Column five gives an optional logicaladdress name. Column six contains the logical pneumonic for theoperation code. Column seven contains the logical address nameassociated with column three. Column eight contains relevant programmercomments. As discussed in the section entitled PROCESSOR AND DISPLAY,the instruction set is basically that described in the referenced patentissued to France Rode et al. In addition, a second set of input/outputinstructions is utilized. These are substantially as described asInstruction Set 2 in the referenced patent of Freddie W. Wenninger etal. A few changes have been made to this instruction set in the presentembodiment. Specifically, certain instructions have been relabeled asTG1, TG2 . . . and LI0 etc. These control the "tiger" and "lion" linesfor control of the present device. Other minor variations are explainedby the relevant programmer comments below.

I claim:
 1. Electro-optical distance measuring apparatus comprising:alight source for producing a light beam signal having an intensity whchvaries at a predetermined repetition rate; a beam-splitter disposed inthe path of the light beam signal for producing first and second lightbeams; beam transmitting optical means disposed to receive the firstbeam for transmitting the first beam to a remote reflector spaced awayfrom the apparatus the distance to be measured; reference optical meansdisposed to receive the second beam for transmitting the second beamover a reference light path; alternator means disposed in the paths ofthe first and second light beams for alternately and cyclically blockingand passing the first and second light beams in phase opposition;receiver optical means disposed to receive the first light beamreflected from the reflector and the second light beam transmitted overthe reference light path for producing first and second electricalsignals in response to the first and second light beams respectively;electrical circuit means coupled to said receiver optical means forgenerating an output signal in response to the phase relationshipbetween the first and second electrical signals; amplifier means havinga variable amplification coupled to said receiver optical means foramplifying the first and second electrical signals by a selectedamplification factor providing first and second amplified signalsrespectively; range means coupled to said amplifier means for providinga first range signal in response to the first amplified signal attaininga value outside a first predetermined range and a second range inresponse to the second amplified signal attaining a value outside asecond predetermined range, the first range being larger than andincluding the values of the second range; comparator means coupled tosaid receiver optical means for producing a balance signal in responseto a difference between the values of the first and second electricalsignals; light regulator means coupled to said comparator means forregulating the intensity of one of the first and second light beams inresponse to the balance signal for substantially equalizing the valuesof the first and second electrical signals; and control means coupled tosaid range means and to said amplifier means for adjusting the variableamplification in response to the second range signal for altering thevalue of the second electrical signal to substantially equal apredefined value.
 2. Electro-optical distance measuring apparatus as inclaim 1 comprising means coupled to said electrical circuit means and tosaid range means for inhibiting the generation of the output signal inresponse to a first range signal being present.
 3. Electro-opticaldistance measuring apparatus as in claim 2 wherein said light regulatormeans is coupled to said range means and is inhibited from varying theintensity of the one light beam in the presence of the first rangesignal.
 4. A method of controlling the operation of an electro-opticaldistance measuring apparatus having a photo-sensitive receiver forproducing first and second electrical signals in response to first andsecond light beams respectively, comprising the steps of:amplifying thefirst and second electrical signals by a selected amplification factor;producing a first detect signal in response to the first electricalsignal attaining a value outside a first predetermined range; producinga second detect signal in response to the second electrical signalattaining a value outside a second predetermined range, the first rangebeing larger and including the values of the second range; selectablyregulating the intensity of one of the light beams for substantiallyequalizing the values of the first and second electrical signals; andselecting the amplification factor in response to the presence of asecond detect signal to alter the value of the second electrical signalto substantially equal a predefined value.
 5. A method as in claim 4comprising the steps of:generating an output signal indicative of themeasured distance in response to the phase relationship between thefirst and second electrical signals; and inhibiting the generation ofthe output signal in response to the presence of a first detect signal.6. A method as in claim 5 comprising the step of:inhibiting variation ofthe regulation of the intensity of the one light beam in response to thepresence of the first detect signal.